Shani Wallis, TunnelTalk

TBM tunnelling has made the first careful move into the soft saturated soils under Amsterdam for construction of the city's new North-South Metro Line running tunnels.

Santa Barbara dedication

After a recent dedication ceremony to Santa Barbara, the adopted patron saint of tunnellers and tunnelling, the first of two 6.83m diameter Herrenknecht Mixshields started its initial 730m journey from the working shaft in front of the city's main Centraal railway station to the first underground station box at Rokin. To date it has built about 20 rings of 1.5m wide segmental lining. A regular progress of about 8m/day is anticipated once space is available in the shaft for the last two gantries of the backup to be attached and the familiar routine is established.

Start of TBM tunnelling in a nervous time for all involved as well as for the citizens of Amsterdam. Construction of the metro system in the city has been plagued by problems and strife since the first underground line was constructed in the 1970s. Violent protests accompanied demolition of old historic buildings to make way for open cut construction of the existing East-West underground line. TBM tunnelling was adopted during planning and design of the North-South Line in the 1990s to reduce surface disruption to a minimum but damage to the famous fabric of the ancient and beloved Dutch capital became an equal cause for alarm and concern.

The North-South Line beneath the heart of Amsterdam

The soil on which the old buildings are founded comprises layers of highly permeable sands and soft silts with layers of compressible peat and organic materials between deposits of stiffer clays. On these soft soils, buildings, some several centuries old, are founded on wooden piles or rafts which are highly sensitive to any movement of the fine soils or lowering of the natural watertable, rising to just below the surface of the famous canalled city.

Slurry shield tunnelling is specified for excavation of the new metro running tunnels as the most advanced method of controlling excavation of the soft, fine materials and the two Mixshields ordered from Herrenknecht for the project by Saturn, the construction consortium comprising Züblin of Germany and local partner Dura Vermeer, are fitted with the most up-to-date computer controls and construction facilities to control excavation and avoid ground loss settlement.

But concerns run high. Major ground loss problems have been encountered on excavation of the open-cut station boxes. In early 2007 cracks appeared in a building adjacent to a station access excavation when water and soil began flowing into the box area through a breach in the sheet piled support wall.

A much more serious situation occurred at the Vijzelgracht station box excavation in June and in September 2008 when large volumes of ground were lost through failures in the slurry diaphragm support walls. The cause of the first event was the accidental leaving behind of a cover plate. The second was caused by a pocket of liquid bentonite left in a slurry panel that failed under pressure from the exterior ground water and caused major ground loss before flows could be stemmed. Several seriously damaged buildings adjacent to the station box failures had to be evacuated and remain so today with their ultimate fate still unknown.

With substantial amounts of the Rokin and Vijzelgracht stations boxes still to be excavated , ground freezing has been installed to ensure further control.

Ground freezing is also installed for launch of the first two TBM drives. A 1.5m thick x 85m long frozen collar has been installed horizontally from the working shaft to support advance of the two Mixshields as they pass through the tunnel eyes in the sheet-piled cofferdam built into the base of the Natte Damrak canal. The ground freezing canopy will protect advance of the TBMs until they pass under the road and on under Damrak square.

With the first TBM - named Gravin, which is a play on the word that means both 'Dutchess' and 'Digger' in Dutch - now into its drive, the second machine will start in about a month's time to maintain a stagger between the leading and trailing tunnels. Once both machines are in the Rokin station box, they will be lifted and transported south to a working shaft at Scheldeplein near the RAI/Euopaplein station box. From here they will drive the 950m, 560m and 880m twin running tunnels to Ceintuurbaan, Vijzelgracht and Rokin stations respectively, being walked through the 200m long excavated boxes as they progress.

Ground freezing supports launch of the TBMs for double security

For the first two drives from Natte Damrak to Rokin station, the slurry pipelines connect to the separation plant set up behind the main Centraal railway station. From here separated soil is taken away by barge for onward disposal. When the machines move to the Scheldeplein working shaft in the south, the separation plant will be located on the Oudekerkerdijk park space and material will be removed by barge on the Amstel River.

The new 9.8km long metro line for Amsterdam is a Combination of many different construction techniques. Where the two Mixshield slurry TBMs will excavated twin tunnels for a total 3,120m between four open-cut underground stations, the 2.8km from Buikstotermeerplein station in the north is on the surface before going below grade into an immersed tube crossing of the Ij River. To the south, the RAI/Europaplein station is also subsurface in an open cut excavation and the underground alignment transitions to an at grade end station at Zuid/WTC (World Trade Centre). Directly under the main Centraal Station building is a complex piece of civil engineering that not only creates a metro station and the connecting route of the alignment in the underground space but also replaces all the wooden piles on which the station building was founded more than a hundred years ago. The excavation work has progressed partly from the TBM working shaft at Natte Damrak.

Excavation below decks of the Centraal metro station

Excavation of the metro station in front of the main station, was carried out in the wet under a surface reinstatement decking until the base slab was tremmied in and the watertight box dewatered. The metro route under the main station is yet to be completed and will be built as an immersed tube floated in to the space and lowered into position. . In the interim, the main slurry circulation pipelines run through the opened space under the main station to link the TBMs and the separation plant on the embankment of the Ij River behind the main station.

Tunnelling and open cut excavation of station boxes in the waterlogged soils beneath Amsterdam was recognized by engineers to require careful control and management. Within the design consortium, engineering firm Witteveen + Bos was charged with completing the geological investigations; predicting ground behaviour; surveying all the structures within the zone of influence; and developing a project-wide monitoring system, the set of trigger levels, and the mitigation measures needed to react to any excessive ground movement triggers.

The firm has developed a sophisticated Geographical Information System (GSI) that links all the data collected from the monitoring systems to the engineering control stations on the project as well as to the control cabin of the TBMs. The system prototype was first used on the Hubertus road tunnel in The Hague where a 10.53m diameter Herrenknecht Mixshield excavated the twin-tube tunnel.

Excavation of the metro station in front the main Centraal railway station

As part of the GIS system, the TBM operator becomes an integral player in the overall ground control management, elevating the responsibility and skill required of a TBM operator. Since construction of the Hubertus tunnel in 2006-08, the GIS system has been upgraded and advanced with more sophisticated electronic and computer based systems.

In the fight against ground loss and surface settlement damage to buildings, Witteneen + Bos has also adopted several support systems including ground freezing and compensation grouting. Grout is injection through preinstalled horizontal tube-a-manchette arrays extending from strategically located access shafts to counteract any excessive ground loss or surface settlement readings.

All these techniques are employed in Amsterdam to do the utmost to protect the familiar structures of the city during development of vital infrastructure services beneath its streets and buildings but the process is expensive. The cost of the North-South Metro Line has risen from its € 800 million budget at time of approval for construction in 1996 to a reported current estimate of € 2.4 billion. With the multiplying cost increases comes time overruns and there are few firm prediction of when trains will be running on the new line.

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